The invention relates to a circuit for supplying power to a network termination unit of a message transmission system, which is connected to a central station via a subscriber line. A local power supply voltage source, which is arranged in the network termination unit and which supplies power during a normal operating state, is provided as well as a remote power supply source, which is arranged in the central station and which supplies power during an emergency operating state in the case of a failure or malfunctioning of the local power supply voltage source. The network termination unit includes a direct current converter with a transformer and a clocked switch. The primary winding of the transformer is connected via the clocked switch to the local power supply voltage source.
In a service-integrating digital message transmission system (ISDN), the subscriber neighborhood is formed essentially by a network termination unit, an S-interface and subscriber terminals. Power to these terminals is supplied in a normal operating state from the network termination via a local supply voltage and the S-interface. The available power enables operation of the terminals connected to the network termination.
If the local supply voltage, which for safety reasons is typically 24 VDC or 20 VAC, fails or decreases below an acceptable level, then the network termination automatically switches into an emergency operating state, wherein the essential functions of the network termination are maintained by remotely supplied power via a remote supply voltage source arranged at a central location, for example in an exchange. The remote supply voltage is thereby supplied via the subscriber line that exists between the central location and the network termination, wherein only a relatively small amount of power is available in the emergency operating state. The remote supply voltage is typically xc2x160 VDC.
Such power supplies are mainly used with ISDN systems, but can also be employed with other transmission systems, such as for example ADSL, HDSL, pair-gain systems (PGS) and the like.
The voltage supplied by the local supply power source as well as from the remote supply voltage source is converted by direct current converters to the values required for the various operating states.
To eliminate supply interruptions during switching from the normal to the emergency operating state, conventional current supply circuits have at least two separate direct current converters or a converter with two separate primary circuits which are arranged in such a way that when the local supply fails, the remote supply is available without interruption. For this purpose, two direct current converters or at least two separate primary circuits have to continuously and in parallel supply the output voltage. Each direct current converter or primary circuit, however, requires a separate control and a corresponding separate transformer winding which take up a relatively large space compared to the other components, making miniaturization of the circuit difficult. In addition, using two direct current converters increases the manufacturing costs.
It is therefore an object of the invention to provide a compact circuit of the aforedescribed type which uses relatively little space and has low manufacturing costs.
This is achieved according to the invention in that a terminal of the primary winding is connected via a first controllable switch with a wire of the subscriber line.
If the local power supply voltage source fails or has insufficient voltage, then the remote supply voltage can be connected via the controllable switch directly with the primary winding of the direct current converter-transformer, so that the remote supply voltage can be converted using one and the same primary circuit, making a second direct current converter unnecessary.
According to another embodiment of the invention, the terminal of the primary winding connected with the first controllable switch can be connected via a second controllable switch with a buffer capacitor.
In this way, the network termination unit can be switched from the normal operating state, in which all terminals are supplied by the network termination, unit into the emergency operating state, in which only the most important operating functions of the network termination unit are maintained, using only a single direct current converter and/or only a single primary circuit and only a single transformer, without encountering a supply bottleneck. The charge stored in the buffer capacitor is thereby capable of supplying to the primary winding of the transformer of the direct current converter the energy necessary to switch into the emergency operating state.
According to another embodiment of the invention, the first and second controllable switch can be formed by field effect transistors, which require only a small control energy.
According to another embodiment of the invention, the buffer capacitor can be connected via a charging branch with a charging voltage source. The buffer capacitor is then continuously charged to compensate for losses in the capacitor.
According to another embodiment of the invention, the charging branch can be formed by a charging resistor which is connected with the subscriber line preferably via a blocking diode.
The charging resistor limits the charging current to the maximum allowable value of the current of the remote supply voltage.
According to another embodiment of the invention, a voltage monitoring unit can be provided whose inputs are connected with the local power supply voltage source, and an output of the voltage monitoring unit can be connected with a unit for controlling the first and second controllable switch.
The voltage monitoring unit monitors the voltage supplied by the local power supply voltage source. If this voltage source fails or the voltage drops below a minimum voltage, a control signal is supplied that controls the second controllable switch, whereby the buffer capacitor is discharged into the primary winding of the transformer. In this way, energy required for switching from the normal operation to the emergency operation is provided during the switch-over time.
According to another feature of the invention, an additional voltage monitoring unit can be provided, whose inputs are connected with the subscriber line, whereby an output of the additional voltage monitoring unit is connected with the unit that controls the first and the second controllable switch.
The additional voltage monitoring unit can continuously monitor the remote supply voltage which is required for transforming the network termination into the emergency operating state.
In a method for switching a network termination unit from the normal operating state into a remotely supplied state by using a circuit according to the invention, it is provided that the amplitude of the local supply voltage is continuously measured and compared with a predetermined minimum value, and that when the predetermined minimum value is underrun, the first controllable switch is closed and the network termination unit is switched from a normal operating state into a remotely supplied state, which preferably represents an emergency operating state.
By closing the first controllable switch, the potential of the remote supply voltage is supplied to the primary winding, which now continues the voltage conversion. Accordingly, the network termination unit is then remotely supplied via the same direct current converter which converts in the normal operating states the local power supply voltage source. If the network termination and the devices connected thereto consume only a small amount of power, then the normal operation can be maintained when switching into the remotely supplied state. Otherwise, the system has to be switched into the emergency operating state.
According to another method for transforming a network termination unit from the normal operating state into the remotely supplied state by using a circuit according to the invention, it is provided that the amplitude of the local supply voltage is continuously measured and compared with a predetermined minimum value, and that when the predetermined minimum value is underrun, the second controllable switch is closed and the network termination unit is switched from a normal operating state into the remotely supplied state, which preferably represents an emergency operating state, wherein during the switch-over time the charge stored in the buffer capacitor is at least partially supplied to the primary winding of the transformer, and that thereafter the first controllable switch is closed.
Discharging the buffer capacitor into the primary winding of the transformer by closing the second controllable switch makes it possible to maintain the supply voltage for the network termination unit during the switch-over into the emergency operating state.
According to another embodiment of the invention, the remote supply voltage can be decreased in the normal operating state to a predetermined value and increased to its full value when the local power supply voltage source fails or has an insufficient voltage or for recharging the buffer capacitor.
In this way, the remote supply voltage can be kept low in the normal operating state and increased only to the required value when needed. This, on the one hand, reduces the energy consumption in the subscriber line due to an excessive remote supply voltage and, on the other hand, reduces the danger associated by a high remote supply voltage.
According to yet another embodiment of the invention, the subscriber line can be forcibly switched into the emergency operating state for maintenance and test purposes and thereafter returned to the normal operating state. In this way, the functionality of the circuit of the invention can be checked.